Insulin increases diacylglycerol (DAG) production and activates protein kinase C (PKC) in a variety of target tissues. This activation may be important for glucose transport. As glucose transport is rate-limiting for peripheral glucose utilization, defects in DAG-PKC signalling may cause insulin resistance. Further, since phorbol ester-induced activation and subsequent downregulation of PKC inhibits insulin-induced glucose transport in rat or mouse adipocytes and muscle tissues, sustained PKC activation and consequent downregulation of PKC by prolonged stimulation by insulin and/or elevated extracellular glucose levels may also inhibit insulin-stimulated glucose transport and cause insulin resistance. Also, since antisense DNA targeted against PKC MRNA downregulates PKC and inhibits insulin-stimulated glucose transport, defective production of PKC may cause insulin resistance. The major interrelated hypotheses that will be examined are: (a) DAG-PKC signalling is important for insulin-stimulated glucose transport; (b) defective DAG-PKC signalling may impair glucose transport and cause insulin resistance; and (c) defective DAG-PKC signalling may result from PKC depletion caused either by decreased PKC synthesis or prolonged PKC activation and turnover by insulin and/or elevated extracellular glucose levels. Implicit in the evaluation of the last hypothesis is the need to determine whether activation and subsequent degradation of PKC, resulting from prolonged insulin or glucose treatment, is attended by compensatory increases in PKC production. The specific questions that the present proposal will address are: 1. To what extent are PKC levels and specific PKC isozymes diminished during acute or prolonged treatment with phorbol esters, PKC antisense DNA, glucose and/or insulin? 2. Do these decreases in PKC cause defects in insulin-stimulated glucose transport? 3. Are there specific types and amounts of PKC that are required for insulin-stimulated glucose transport? 4. Are decreases in tissue PKC levels during glucose and/or insulin treatment attended by changes in PKC production and/or PKC MRNA levels?